Central nervous system (CNS) involvement alone, it appears that rabies virus may be a systemic infection that causes pneumonitis, encephalitis, and myocarditis. The vast majority of infections which exhibit viremic spread fail to involve the CNS. Once virus reaches the CNS, disease results only if the virus spreads and if this spread directly or indirectly causes functional disturbances. Damage can result from cellular destruction, as seen in polio virus infection or lymphocytic chorio meningitis virus infection of adult mice or it can result from cellular dysfunction in absence of cellular pathology, as seen with rabies virus infection. The increased susceptibility of pregnant women to certain viral infections appears to be causally related to elevated hormone levels during this predicament, since hormones increase the susceptibility of animals to a variety of viral infections. This effect, in turn, presumably reflects the inhibitory influence of hormones on interferon producing capacities of cells, which has been demonstrated both in vivo and in vitro.